Gotcha.
Since file retrieval does not work would it be possible to clarify the forum rules
Don’t upload your files to external servers and post the links, these files will be deleted after a while and the thread will become useless, attach your files in the post. Moderators will delete any posts that includes files uploaded to file sharing servers like Rapidshare, 4shared etc.
Does it mean ANY external server or just file sharing external services? If external images are completely prohibited and file retrieval does not work, maybe the 'insert image by URL' should be disabled altogether?